Kokushibo vs Tanjiro is the worst possible thing the movies could do.
Having Kokushibo and Tanjiro cross paths would undermine Kokushibo's entire character arc by justifying his motives and philosophy.
I know it's literally just for power-scaling reasons but you already have your answers.
When Kokushibo met Yoriichi, he almost fucking died.
You're going to get your sun vs moon breathing dick-measuring contest with this page anyway. Just like every other brief panel, Ufotable will extend it into a proper fight scene, even though it probably should be a clear curb stomp (they did the same thing with Akaza vs Kyojuro), and it will be breathtakingly tragic.
(Note: Yoriichi's death also robs Kokushibo, of what he perceived to be, an honourable death a remenant of his humanity and personal goals as a samurai. This relates to his reaction to Akaza's death but that's another post and more of a personal headcanon)
Michikatsu becomes a demon primarily motivated by his envy of Yoriichi, all his secondary motives are more or less tied to that.
But one aspect that I wanted to highlight is Michikatsu's superiority complex (which often goes hand-in-hand with an inferiority complex; some even argue you can't have one without the other, but that's a diff conv). As a human he's already chipping away at his own life and desires just to try and outdo Yoriichi, who's completely oblivious to this invisible competition as far as we can tell.
Michikatsu believes that the current generation of demon slayers is the greatest to ever exist. He believes that no one can master sun or moon breathing techniques, and that there isn't enough time to train and finally best Yoriichi in this one-sided rivalry.
He just was incapable of seeing a future past his own, despite his brother's mantra about people mastering their skills and unshakable optimism for the future.
All this compounded with his desire to be better than Yoriichi made him readily accept Muzan's offer. Time was ticking with every single marked slayer dying.
The fight with the Hashira + Genya, is a far better way to disprove Kokushibo's philosophy more than any hypothetical battle that involves Tanjiro.
He's constantly noting how the current gen slayers have vastly improved over his supposed "golden generation". They're using forms he's unaware of (I feel like it's kind of mirroring Akaza vs Giyuu's fight)
Kokushibo is never going to reach this realisation of just how WRONG he was if he's fighting Tanjiro (who we know is objectively weaker than Yoriichi anyway). With the Hashira (+ Genya) it's different, the slayers he faced wield derivative forms which Kokushibo automatically dismisses as inferior imitations of Yoriichi's talent. Yet he's either struggling or in awe at what they're accomplishig. There are too many panels to show as examples tbh.
Yoriichi's vision came to pass. The future generation far surpassed their predecessors. They extended the forms Kokushibo was once familiar with and together with their combined strengths were able to take down Upper Moon 1.
I would never exchange this narrative irony for another sun vs moon fight WE'RE ALREADY GETTING IT.
In Kokushibo's final moments he realises how pointless his sacrifices and demon life truly were.
He still has no successors for his technique while the inferior breathing styles lived on for centuries, evolving with each generation.
He ended his family line by his own hands.
He exchanged his samurai life and his family (in the most charitable translation he views his family as a blessing but idk) for joining the demon slayer corps in a futile attempt to one up his brother. When that failed and time was running out, he traded away the rest of his humanity to become a demon, robbing himself of an honourable death and leaving behind no legacy.
It's very bluntly stated as well
I don't think Kokushibo would have come to his bitter conclusion if he'd fought a sun breath instead of the Hashira who are using techniques he brushed off centuries ago as inferior to his and his twin's.
Fighting Tanjiro would leave Kokushibo feeling vindicated. Despite learning sun breathing Tanjiro can't master Yoriichi's techniques, nor is he as talented as Yoriichi, and since he's marked will die young.
Boiling down Yoriichi's legacy and influence to just a pair of earrings and a specific technique does a massive disservice to Yoriichi himself, and by extension to Kokushibo's entire tragedy.
it works a lot better with Muzan since Muzan tried his best (via Kokushibo) to snuff out the sun breathing technique and until the story he still feels Yoriichi's effect on him.
Anyway basically...Kokushibo is the personification of Dostoevsky's quote.
The world moves forward with or without you. Kokushibo spent centuries running away from something Yoriichi just accepted. In the end Yoriichi's ideas prevailed while Kokushibo was lost to history.
For a very simple story it's annoying to see this assertation time and time again.
